In the field of mobile communications, large-capacity and high-speed communication techniques have been developed. Among these, MIMO (Multi-Input Multi-Output) techniques in which multiple transmitting antennas and multiple receiving antennas are used have been put into practical use. The MIMO antennas need to have reduced correlation so as to ensure independent communication channels.
Many cell phone base stations adopt a dual-polarized antenna using two orthogonal polarizations such as vertical and horizontal polarizations or +45 degree and −45 degree polarizations. The dual-polarized antenna has advantages such as implementation of antenna branches for two systems, i.e., two-branch MIMO communications, low correlation between the two antennas, and size reduction attributed to the fact that antennas can be installed close together.
To give examples of currently available base station antennas, there are a sector antenna capable of covering a sector-shaped area, an omnidirectional antenna capable of covering a circular area, a planar antenna or Yagi antenna capable of covering an area at a certain spot, etc. Many of these antennas can use both the vertical polarization and the horizontal polarization.
Most of the base station antennas using both the vertical and horizontal polarizations are array antennas composed of dipole elements. The antenna of this type emits a vertical polarization from a dipole element installed vertically to the ground and emits a horizontal polarization from the one installed horizontally to the ground. The above sector antenna, omnidirectional antenna, and planar antenna, etc. can be designed in various ways by changing the dipole element array. Note that the Yagi antenna is not an array antenna and instead, has multiple parasitic elements arrayed in front of dipole elements.
These dual-polarized antennas are expected to be as compact as possible so as to reduce wind load and improve appearance, for example. For that purpose, numerous trials are ongoing to reduce size and to make such a dual-polarized antennas thinners, but these efforts seem to have nearly reached their limits.
On the other hand, periodic antenna structures, incorporating metamaterial, have been studied and tentatively applied to mobile communication antennas. The metamaterial antennas show characteristics unexpected from common antennas and also allow size reduction. Their applications to mobile communication antennas are therefore promising, but only a few applications have been reported.
A leaky-wave antenna using a CRLH (Composite Right/Left-Handed) transmission line is known as such a metamaterial antenna. The leaky-wave antenna emits leaky waves forward in right-handed bands and emits leaky waves backward in left-handed bands. Advantageously, this provides wide-angle beam scanning
Non-Patent Literature 1 proposes a CRLH leaky-wave antenna with microstrip transmission lines. Non-Patent Literature 2 proposes a CRLH leaky-wave antenna with a waveguide.